Answer 1

Here is how I would do it İsmail ÖZBAKIR :

else { return -1; }

int main() {
int answer = factorial(-1);
if (answer <= -1) {
cout << "Negative number invalid";
}
else {
cout << answer << endl;
}

Answer 2

You can't do that using recursion, because in C++ you can't return multiple types. You would have to throw an exception error and catch it when you call it in main.

Answer 3

You still need to return an integer. You can't use cout with a function using cout; that's like doing this:

cout << cout << ...

Answer 4

Return -1 if input is under 0.